Positive feed drills are well known for drilling holes through hard substrates such as steel and titanium. Positive feed drills generally include a spindle that, in addition to rotating, also advances toward the substrate being drilled. Conventional applications for positive feed drill include, for example, aircraft fuel tanks. When using positive feed drills, coolant or lubricant is ordinarily delivered by a mist lubricator to reduce the heat generated through the friction of drilling and also to provide lubrication. Conventional mist lubricators may be manually started or may automatically start when the positive feed drill is turned on. Conventionally, coolant or lubricant is delivered from a reservoir through a hose where it enters a union attached to a hollow rotating spindle of the positive feed drill. The coolant or lubricant mist travels through the spindle until it reaches a cutter associated with the positive feed drill. This procedure allows cooling or lubrication at the interface of the cutter with the material being drilled.
Conventionally, when the cutter breaks through the substrate that is being drilled, a user must either manually terminate the lubrication mist or, in conventional automatic systems, completely shut off the drill. These conventional techniques create several problems. For example, when the cutter breaks through the substrate, coolant or lubricant may spray through the substrate into the area behind the substrate being drilled. In some cases, the coolant or lubricant may spray into an aircraft fuselage or fuel tank, which are typical applications for positive feed drills. This problem is exacerbated in applications in which several layers of material having air gaps between the layers are drilled. Such applications are common in aircraft manufacturing. The spraying of coolant or lubricant into these areas causes contamination because, for example, many of the types of fluids used as coolants or lubricants do not mix well with adhesives used to seal fuel tanks. Furthermore, spraying of coolant or lubricant into aircraft fuel tanks could lead to contamination of the aircraft fuel. Therefore, conventionally, after use of such a tool, the inside of the device being drilled must be cleaned, which increases the expense associated with the drilling procedure. Other applications in which it is undesirable for coolant or a lubricant to feed into the drilled area also suffer similar disadvantages.
In the past, efforts to address these problems have focused on employee training, encouraging operators of positive feed drills to either turn off the positive feed drills or manually terminate coolant or lubricant flow immediately upon penetration of the cutter into a drilled region. However, accurately identifying the point at which the cutter breaks through a drilled substrate is often difficult and often requires expensive equipment; and therefore, the results achieved through such attempts to address the above-described problems associated with providing lubricant to a cutter are somewhat lacking.